NEW ORLEANS (AP) _ Evolution is just as much a religious belief as creationism, says a former state senator whose legislation requiring their teaching side-by-side in public schools failed a constitutional test.

The nation's only law requiring the teaching of creationism along with evolution was struck down Monday by the 5th U.S. Circuit Court of Appeals.

The law, enacted in 1981, was never enforced and was similar to an Arkansas law struck down in 1982.

Attorney General William Guste said he would have to study the opinion before deciding whether to go on to the U.S. Supreme Court.

The three-judge appellate panel upheld and expanded on U.S. District Judge Adrian Duplantier's finding in January that the law was unconstitutional.

''The act's intended effect is to discredit evolution by counterbalancing its teaching at every turn with the teaching of creationism, a religious belief,'' the 5th Circuit said.

''They say that creation science is a religious belief, but evolution also is a religious belief,'' former state Sen. Bill Keith said Monday night.

''It is the cardinal belief of Hindus, Buddhists, Unitarians and theological liberals,'' said Keith, a 50-year-old Baptist minister who was defeated in 1983 in his bid for re-election and now lives in Jefferson, Texas.

''The obvious question is - if the judiciary considers creation science a religious teaching, why is there no concern among the judiciary that evolution also could be categorized as a religious belief?''

Creationism is the theory that the Earth and everything on it was created all at once thousands of years ago, as outlined in the Bible's Book of Genesis.

The theory of evolution holds that life began billions of years ago and that man gradually developed from simpler forms. The 5th Circuit said, ''Not only does the Act fail to promote academic freedom, it fails to promote creation science as a genuine academic interest. If primarily concerned with the advancement of creation science, the Act, it certainly appears to us, would have required its teaching irrespective of whether evolution was taught.''

Martha Kegel, New Orleans director of the American Civil Liberties Union, which fought the law, was jubilant.

''This decision puts the Louisiana creationism law out of its four-year misery,'' she said. ''With the decision today, the federal appeals court made it very clear, abundantly clear, that creationism is religion, that it is not science, and that the courts will not tolerate government promotion of religion in the science classroom.''

Last week, the Louisiana House killed a bill that would have forbidden the state from paying any more for appeals.

State Rep. Peppi Bruneau, who handled the bill, said, ''The 5th Circuit has done what the Legislature should have done.''

Lawyers for the state had hoped the 5th Circuit would order a full trial of the case, but the court responded with a 15-page opinion.

''Nothing in our opinion today should be taken to reflect adversely against creation-science, either as a religious belief or as a scientific theory,'' the court said. ''Rather, we seek to give effect to the First Amendment requirement that demands that no law be enacted favoring any particular religious belief or doctrine.''

The court said the law, called ''Balanced Treatment for Creation-Science and Evolution-Science in Public School Instruction'' failed three tests:

-''Whether the statute has a secular legislative purpose.''

-''Whether the principal or primary effect of the statute advances or inhibits religion.''

-''And whether the statute fosters an excessive entanglement with religion.''